A gas explosion is one of the main hazards of underground utility tunnels (UUTs), and the explosion overpressure can have catastrophic consequences. A pressure relief opening (PRO) can be an effective method of reducing the risk of an explosion. However, most existing UUTs in China are not configurated with a PRO. Therefore, the present study proposes a risk-based PRO design to explore the optimal layout for UUTs. Both dispersion and explosion simulations using Flame ACceleration Simulator (FLACS) were conducted to calculate the maximum overpressures, and then probabilistic studies were applied for risk analysis. The FLACS simulation results show that PROs can significantly reduce the maximum explosion overpressure from 2.23 bar to 1.97 bar. An ALARP (As Low As Reasonably Practicable) concept was applied to provide a risk reference for the design of a PRO in UUTs.